The present invention relates to a process and device for bleaching cellulose pulp with molecular oxygen under alkaline conditions and at an elevated temperature.
The use of oxygen in the bleaching of cellulose has been an object of increasing interest in recent years. Oxygen-alkali bleaching has considerable advantages when compared with the previous processes using chlorine chemicals.
According to one known process, the oxygen-alkali treatment of pulp is carried out so that alkali-treated pulp with a fiber concentration of some 25% is fed into a tank which contains oxygen gas.
According to another process, oxygen gas is fed into a tank which contains cellulose fibers in an alkali solution.
It has also been suggested that the oxygen gas treatment be carried out by displacing liquid from alkali-treated pulp by means of pressurized oxygen gas.
All the above, previously known processes require large reaction tanks. The velocity of the bleaching reaction is dependent on the temperature. For this reason it is desirable to maintain a relatively high temperature in the tank. The dissolving of oxygen in the alkali solution decreases when the temperature raises, whereby the treatment period is lengthened. The concentration of oxygen increases along with pressure, so that when a short treatment period is the object, a high pressure must be used, which results in high equipment costs.
The object of the present invention is to provide a process and device for bleaching cellulose pulp with molecular oxygen, without the above disadvantages.